Plant shelters protect young plants against early frost, damaging wind, unseasonably cool weather, heavy rain, hail, excessive sunlight, and insects and other pests. In addition, heat and moisture are conserved within the shelters to provide an environment that promotes the growth of young plants. The protections and controlled environment allow for earlier plantings that significantly extend growing seasons, especially in cold climates.
Eventually the shelters are removed to allow the young plants to grow to maturity. However, the abrupt removal of the plant shelters can subject the young plants to considerable stress. Accordingly, some plant shelter designs control ventilation for gradually acclimating or "hardening" young plants to their surroundings.
For example, U.S. Pat. No. 74,879 issued to R. M. Bartlett more than a century ago discloses a plant shelter having a conical shell made from pasteboard or thin wood and having two sets of openings. One set of large openings is covered with an oil paper, muslin, or glass for admitting light into the shelter. Another set of small openings is covered by a rotatable collar having a similar set of openings that can be variously aligned with or offset from the set of small openings in the cone-shaped hood for controlling ventilation within the shelter. Additional ventilation is provided by raising the hood on a pole as the protected plants continue to grow.
Most subsequent plant shelter designs for controlling ventilation have generally followed Bartlett's approach of providing rigid relatively movable parts for adjusting opening sizes. Examples of such designs are found in U.S. Pat. No. 3,093,930 to Witkowski; U.S. Pat. No. 3,226,881 to Garrett; U.S. Pat. No. 4,392,326 to Boria; and U.S. Pat. No. 4,711,051 to Fujimoto. The multiple parts of these designs add cost, and the varying size openings also affect the amount of light admitted into the shelters.
An alternative approach to controlling ventilation in plant shelters is disclosed in U.S. Pat. No. 3,214,865 to Rosenvold et al. The alternative plant shelter is formed from a transparent or translucent plastic material that is perforated for forming a series of knockout sections. Ventilation openings are formed by removing the knockout sections. However, once formed, the ventilation openings are permanent, and a new shelter is required to reduce ventilation.
U.S. Pat. No. 1,814,339 to Sato discloses a plant shelter that provides for separately controlling the admission of air and light into the shelter. Ventilation is controlled in the usual way with an adjustable cap covering an opening. The shelter is made from glass for admitting light. However, one side of the shelter is made thicker with corrugations that are intended to diffuse light and heat. The corrugated side can be rotated toward or away from the source of light to block varying amounts of light from entering the shelter. In addition to requiring multiple rigid parts for controlling ventilation, the glass material is heavy and subject to breakage.